Conventionally, in hot rolling lines of metal blocks comprised, for example, of steel, aluminum, copper and the like, the metal blocks to be rolled are heated, rough rolled and finish rolled one by one, so as to be finished into a plate having a predetermined thickness. Such a rolling system suffers from disadvantages that a poor threading of the rolled material in the finish rolling inevitably causes the line to stop, and a poor shape of fore end portion and rear end portion of the rolled material results in a low yield.
Therefore, it is a recent trend to carry out an endless rolling in which the metal blocks to be rolled are connected at the fore end and rear end portions thereof prior to the finish rolling and continuously conveyed to the hot rolling line. As prior art in this connection, various proposals are disclosed, for example, in Japanese Patent Application Laid-Open Nos. SHO-58-112601, SHO-60-244401, SHO-61-159285, SHO-61-144203, SHO-62-142082, SHO-62-234679, HEI-4-84609, HEI-4-89120, HEI-5-185109, etc.
According to a general practice, the regions of the metal blocks in the vicinity of their respective end portions are clamped and supported by clamps on the inlet side of the rolling equipment and the to-be-joined portion is heated to an elevated temperature and pressed and joined by heating means as the endless rolling of the metal blocks is performed. However, in joining the metal blocks which are passed through the abovementioned processes, there still remain various disadvantages such as those described below, in connection with which there have been demands for improvements.
(1) In butt-joining the metal blocks, it is necessary to precisely align the opposite end portions of the blocks and press them with each other. As shown in FIG. 1, the respective end portions of the preceding metal blocks 1 and the succeeding metal blocks 2 are applied with an alternating magnetic field in the direction of thickness, for example, by heating means 3 such as an induction heating coil. When such end portions are heated to an elevated temperature in a short time, even if the metal blocks are fixed and held by clamps, the end portions may shift vertically upon heating and pressing of the metal blocks. That is to say, the joined portion of the metal blocks reaches 1,300-1,500.degree. C. in temperature and may be partly melted. Such melted portion has a decreased friction coefficient at the joining interface so that even a slight lack of uniformity in the pressing state may result in a vertical shifting of the plates. This may cause either formation of dissatisfactory joining as shown in FIG. 2 or occurrence of buckling. Thus, when the rolling is performed in such a condition, as shown in FIG. 3, the joined portion of the metal blocks may tilt down when it is bit into the rolls, and deformed and threaded into the base plate of the metal blocks, thereby giving rise to formation of excessively thin portions according to an increased number of rolling passage and a resultant breakage of the the plate during the rolling. PA0 (2) The joining apparatus for joining the metal blocks is generally constructed such that a carriage movable on a conveying line of the metal blocks is provided with clamps for fixing the metal blocks, heating means (e.g., an induction heating coil) for heating the metal blocks to an elevated temperature, and pressing means for pressing the end portions of the metal blocks with each other. In this instance, it has been a general practice to cause the heating means to approach the metal blocks only in the use condition and maintain it off the line except during the heating, and such a practice proved to be free from problems. However, the clamps and the pressing means remain mounted on the carriage and hence located on the conveying line of the metal blocks, so that they are subjected to a substantial thermal load and their lives become relatively short. Moreover, the carriage mounting the clamps and the pressing means cannot be attached with table rollers for structural reasons, so that scratches are formed due to the sliding motion of the metal blocks in the carriage when scales are deposited thereon. Further, the maintenance of the clamps and the pressing means cannot be performed except when the transfer of the metal block is stopped or when the rolling is stopped. PA0 (3) In the rolling line provided with a movable-type joining apparatus suitable for joining the metal blocks during the running, there is required an ascending and descending timing control with which the table rollers supporting the metal blocks are moved vertically corresponding to the running of the joining apparatus. A conventional table roller disclosed, for example, in Japanese Patent Application Laid-Open No. HEI-4-367303 lacks in accuracy and reliability as described below. Also, in connection with the driving cylinders for causing the vertical motion of the table rollers, it has been recognized disadvantageous that the hydraulic apparatus has to be large in scale due to a requirement for substantial flow rate of operating oil, thereby giving rise to substantial increase in the power loss and the running cost. PA0 (4) In the pressing process of the metal blocks, the butted portions are protruded with a protruding height of 10-25% of the base plate thickness, which is variable depending upon the pressing amount. Such protruding portion is referred to as a "poor joined portion" hereinafter, including burrs and the like. This may be influential on the pressing force of the rolling mill and the tension of the plate, such that the plate may be broken during rolling, either the rolling mills tend to be injured or the uniformity of the plate thickness tends to become poor as the poor joined portion is bit into the rolls, and fallen foils are likely to be formed.
FIG. 4 shows a construction of the equipment disclosed in the abovementioned patent, and FIG. 5 shows a control mechanism relating to a vertical motion of the table rollers disposed in such equipment.
In FIG. 4 mentioned above, since the preceding metal block 1 and the succeeding metal block 2 are joined with each other while they are conveyed, the metal blocks joining apparatus 4 first assumes a waiting position on the upstream side (left side in FIG. 4) of the conveying line for the metal blocks. And, when the rear end portion of the preceding metal block 1 and the fore end portion of the succeeding metal block 2 reach to the joining apparatus 4, both metal blocks 1, 2 are clamped by pinch rolls p.sub.1, p.sub.2 disposed on the joining apparatus 4 while the joining apparatus 4 runs at the same speed as the conveying speed of the metal blocks, and the rear end portion of the preceding metal block 1 and the fore end portion of the succeeding metal block 2 are heated and pressed until the joining apparatus 4 reaches a predetermined position on the downstream side of the line to complete the joining. On this occasion, the control unit 5 shown in FIG. 5 detects the position of the joining apparatus 4 to operate driving cylinders 7a-7g through an electromagnetic valve 6, respectively, to cause vertical motion of the respective movable table rollers 8a-8g for preventing them from colliding against the joining apparatus 4. In the equipment constructed as above, there is required a control by which the table rollers positioned in front of the joining apparatus 4 are descended when the joining apparatus 4 reaches thereto and the table rollers are ascended immediately after the joining apparatus 4 has passed therethrough. However, there is a problem relating the accuracy and reliability of the electromagnetic valve 6 for performing the control and the vertical motions of the table rollers, so that the joining apparatus 4 and the table rollers 8a-8g may collide against each other. Also, in connection with the driving cylinders 7a-7g for causing the vertical motion of the table rollers 8a-8g, it has been recognized disadvantageous that the hydraulic apparatus has to be large in scale due to a requirement for substantial flow rate of operating oil, and such apparatus has to be continuously operated thereby giving rise to substantial increase in the power loss and the running cost.
In this connection, there has been proposed a pressing method in which the poor joined portion is sandwiched and crushed in the upward and downward directions or a press-cutting method using a cutter (Japanese Patent Application Laid-Open No. SHO-63-160707). Since, however, the poor joined portion is extended in the fallen foil manner by the pressing method, particularly when thin plates are rolled, the plate may be broken from such extended portion during rolling.
On the other hand, in the press-cutting method using a cutter, it is difficult to coincide the position of the cutter to the poor joined portion during running and the life of the cutters is short due to an increased cutting resistance. When, furthermore, the preceding metal block and the succeeding metal block are out of alignment relative to each other as the metal blocks are joined, the cutter may be caught by the non-aligned portion with the result that the plate is broken or the cutter is injured. Therefore, not only the cost for the cutter increases, but also there has been a limitation in connection with improvement in the manufacturing productivity.
Besides, as means for removing the poor joined portion, there have been known hot scarfing method and hot grinding method. However, both of them have been dissatisfactory to perform removal, within a short time of about 1 second, of a surface of the metal blocks at a temperature in excess of 1,000.degree. C. or more, as in the continuous hot rolling aimed at by the invention.